Cochlear implants and other inner ear prostheses are one option to help profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids that just apply an amplified and modified sound signal; a cochlear implant is based on direct electrical stimulation of the acoustic nerve. Typically, a cochlear implant stimulates neural structures in the inner ear electrically in such a way that hearing impressions most similar to normal hearing are obtained.
FIG. 1 shows a section view of an ear with a typical cochlear implant system. A normal ear transmits sounds through the outer ear 101 to the eardrum 102, which moves the bones of the middle ear 103, which in turn excites the cochlea 104. The cochlea 104 includes an upper channel known as the scala vestibuli 105 and a lower channel known as the scala tympani 106, which are connected by the cochlear duct 107. In response to received sounds transmitted by the middle ear 103, the fluid filled scala vestibuli 105 and scala tympani 106 function as a transducer to transmit waves to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Frequency processing seems to change in nature from the basal region of the cochlea, where the highest frequency components of a sound are processed, to the apical regions of the cochlea, where the lowest frequencies are analyzed.
Some persons have partial or full loss of normal sensor/neural hearing. Cochlear implant systems have been developed to overcome this by directly stimulating the user's cochlea 104. A typical cochlear prosthesis essentially includes two parts: the speech processor and the implanted stimulator 108. The speech processor (not shown in FIG. 1) typically includes a microphone, a power supply (batteries) for the overall system and a processor that is used to perform signal processing of the acoustic signal to extract the stimulation parameters. In state-of-the art prostheses, the speech processor is a behind-the-ear (BTE-) device. The implanted stimulator generates the stimulation patterns and conducts them to the nerve tissue by means of an electrode array 110 which usually is positioned in the scala tympani in the inner ear. The connection between speech processor and stimulator is usually established by means of a radio frequency (RF-) link. Note that via the RF-link both stimulation energy and stimulation information are conveyed. Typically, digital data transfer protocols employing bit rates of some hundreds of kBit/s are used.
One example of a standard stimulation strategy for cochlear implants is called “Continuous-Interleaved-Sampling strategy” (CIS), which was developed by B. Wilson (see, for example, Wilson B S, Finley C C, Lawson D T, Wolford R D, Eddington D K, Rabinowitz W M, “Better speech recognition with cochlear implants,” Nature, vol. 352, 236-238, July 1991, incorporated herein by reference in its entirety). Signal processing for CIS in the speech processor typically involves the following steps:
1. Splitting up of the audio frequency range into spectral bands by means of a filter bank,
2. Envelope detection of each filter output signal,
3. Instantaneous nonlinear compression of the envelope signal (map law), and.
4. Adaptation to thresholds (THR) and most comfortable loudness (MCL) levels
According to the tonotopic organization of the cochlea, each stimulation electrode in the scala tympani is associated with a band pass filter of the external filter bank. For stimulation, symmetrical biphasic current pulses are applied. The amplitudes of the stimulation pulses are directly obtained from the compressed envelope signals (step (3) of above). These signals are sampled sequentially, and the stimulation pulses are applied in a strictly non-overlapping sequence. Thus, as a typical CIS-feature, only one stimulation channel is active at one time. The overall stimulation rate is comparatively high. For example, assuming an overall stimulation rate of 18 kpps, and using an 12 channel filter bank, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal.
The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be chosen arbitrarily short, because the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For a 12 channel system with an overall stimulation rate of 18 kpps, the phase duration is 27 μs, which is at the lower limit.
CIS essentially represents envelope information in the individual channels. Temporal cues, e.g., the variations of the envelope signals with the pitch frequency, are presented to some extent. With a Channel Specific Sampling Sequences (CSSS) concept (see, for example, U.S. Pat. No. 6,594,525, “Electrical nerve stimulation based on channel specific sampling sequences,” incorporated herein by reference in its entirety) the amount of temporal information is significantly increased. Temporal variations of the band pass output signals (sometimes designated as “temporal fine structure information”) is represented in the lower frequency range, typically up to about 1 kHz. So a typical stimulation setting may include a mixture of low frequency CSSS channels and high frequency CIS channels. For each CSSS channel, a specific normalized sequence of ultra-high rate stimulation pulses is defined. For stimulation, the zero crossing of the associated band pass filter output is detected, and each zero crossing triggers such a predefined sequence, whereby the sequence is weighted with a factor derived from the instantaneous envelope of the band pass output. Thus, both the envelope and the temporal fine time information is represented in a CSSS stimulation sequence.
To enable a sufficiently high temporal resolution for CSSS, supporting concepts such as “Channel Interaction Compensation (CIC)” for simultaneous stimulation (see, for example, U.S. Pat. No. 6,594,525, entitled “Electrical Nerve Stimulation Based on Channel Specific Sampling Sequences,” incorporated by reference in its entirety) or the “Selected Group (SG)” algorithm (see, for example, U.S. Patent Application Publication No. 20050203589, entitled “Electrical Stimulation of the Acoustic Nerve Based on Selected Groups,” incorporated herein by reference in its entirety) may be utilized.
However, spatial channel interaction may cause distributions of electrical potentials, which could lead to unintentional hearing impressions. For example, let two neighboring stimulation electrodes 1 and 2 generate sequences with CSSS repetition rates of 100 Hz and 200 Hz, respectively. Because of spatial channel interaction, the 200 Hz sequence will distort the 100 Hz sequence at the position close to electrode 1, and could, e.g., lead to a 200 Hz hearing impression (octave failure). Vice versa, the 100 Hz sequence will distort the 200 Hz sequence in the vicinity of electrode 2 and could cause an additional 100 Hz tone which is could be audible. The amount of mutual distortion may depend on the exact phase relationship between the two sequences, and the channel interaction.